1. Field of the Invention
The present invention relates to simulating wafer printing and in particular to supplementing wafer print simulation using optical models with a threshold look-up table (LUT).
2. Description of the Related Art
To fabricate an integrated circuit (IC), a physical representation of the features of the IC, e.g. a layout, is transferred onto a plurality of masks. Note that as used herein, the term “mask” includes “reticles”. The features make up the individual components of the circuit, such as gate electrodes, field oxidation regions, diffusion regions, metal interconnections, and so on. A mask is generally created for each layer of the IC. To create a mask, the data representing the layout for a corresponding IC layer can be input into a device, such as an electron beam machine, which writes IC features onto the mask. Once a mask has been created, the pattern on the mask can be transferred onto the wafer surface using a lithographic process.
Lithography is a process whose input is a mask and whose output includes the printed patterns on a wafer. To facilitate this pattern transfer, a material called photoresist is applied as a thin film to the surface of a wafer layer. The photoresist is exposed to light or some other form of radiation through the mask. This development can remove either exposed portions of the photoresist (for a positive photoresist) or unexposed portions of the photoresist (for negative photoresist). Therefore, subsequent processing of the wafer, which includes etching of the exposed portions of the wafer layer, should replicate the mask pattern in that wafer layer.
To optimize this replication, compute programs can be used to simulate the two-dimensional aerial image that is formed from exposing the mask. Such tools can simulate wafer printing using mask images obtained during mask inspection. These tools typically use optical models to simulate the two-dimensional aerial images (i.e. the wafer images). Based on the predicted aerial image, the mask pattern can be altered, if necessary, to better replicate the mask pattern on the wafer layer.
However, when feature sizes shrink below 90 nm, optical models may not provide sufficiently accurate simulation results. Specifically, the role of photoresist (also called resist) can significantly increase below this dimension, thereby changing the simulation results provided by optical models. Moreover, the next generation resist, i.e. resist for the 193 nm technology node and below, may fail to provide high contrast definition. Current optical models assume that resist at the 248 nm technology node can provide high contrast definition. Therefore, simulations generated using these optical models for the next generation of resist could be inaccurate.
Using a resist model for feature sizes below 90 nm would yield significantly more accurate simulation results than using an optical model. Unfortunately, resist modeling is computationally intensive. Specifically, a user must provide various resist parameters (e.g. photoresist thickness, etch rate, etc.) in addition to stepper parameters (e.g. numerical aperture (NA), wavelength (λ), partial coherence (σ), and illumination type). Therefore, resist modeling is very time consuming, e.g. on the order of 10× slower than optical modeling, thereby making it impractical in a production environment.
To increase the accuracy of the simulation results using an optical model, a threshold could be determined. The threshold refers to the level of light intensity at which the photoresist is activated, thereby printing a feature on the wafer. Unfortunately, a mask house, which is responsible for manufacturing the mask, would typically not have this threshold information.
The threshold of a specific feature (or defect) can be inferred from a neighboring feature. For example, a reference (i.e. a known) feature in the mask image could be simulated-to determine its critical dimension (CD). That CD can then be used to determine the threshold on an associated aerial image. This threshold can then be used in simulating the feature of interest in the same mask image. However, a mask typically does not include such reference features. Therefore, an optical model with a fixed (e.g. a standard) threshold is generally used, thereby resulting in sub-optimal simulation results.
Therefore, a need arises for a commercially viable process that can provide accurate simulation results when feature sizes shrink below 90 nm.